Method of electroplating zinc



Patented Dec. 4, 1951 METHOD OF ELECTROPLATING ZINC Allan E. Chester, Highland Park, and Ray F. Main and Harold F. Hansen, Chicago, 111.; said Main and said Hansen assignors to Acme Steel Company, Chicago, 11]., a corporation of Illinois, and said Chester assignor to Poor & Company, Chicago, 111., a corporation of Delaware No Drawing. Application June 19, 1948, Serial No. 34,129

7 Claims. (Cl. 204-55) This invention relates to an acid zinc-electrolyte and to a method of electrodepositing zinc from acid zinc plating baths. More particularly, the invention relates to the deposition of image bright zinc deposits from acid vzinc electroplating baths. The expression image bright is used herein to describe a deposit or plate which is sufiiciently bright or brilliant to reflect an image.

The ordinary deposit of zinc from an acid zinc electroplating bath is gray and non-reflecting. Although many attempts have been made heretofore to electrodeposit zinc from acid plating baths in such a way as to produce directly a bright zinc deposit, so far as is known no process has been provided which would produce brilliant or image bright zinc plates directly from an acid zinc plating bath without a subsequent chemical treatment of the electrodeposited zinc.

One of the objects of the present invention is to provide a method of electrodepositing zinc from an acid zinc electroplating bath which will produce image bright zinc deposits directly out,

ducing image bright zinc deposits from an acid zinc plating bath in a continuous process.

Still another object is to provide a method of producing image bright zinc deposits from an acid zinc plating bath at elevated temperatures where the conductivity of the bath is higher than at room temperatures.

A further object of the invention is to provide a method of producing image bright zinc deposits from an acid plating bath at high current densities.

An additional object of the invention is to provide a new and improved method of producing brilliant zinc deposits directly out of an acid zinc plating bath over a wide range of current densities', temperatures and agitation.

A further object is to provide a new and improved electrolyte for producing image bright vention it has been found that neither the gluzinc deposits directly from an acid zinc plating bath. Other objects will appear hereinafter.

In accomplishing these objects in accordance with this invention, it has been found that by maintaining the lead content of an aqueous-acid zinc plating bath containing dissolved lead salts below 5 milligrams per liter and by incorporating with said acid zinc bath certain organic addition zinc plates directly without the necessity forany subsequent chemical treatment. The process is especially important in the continuous electrodeposition of zinc, as, for example, in the electroplating of continuous strips or sheets. Image bright zinc deposits may be obtained by the practice of the process over a wide range of current densities and temperatures and with varying amounts of agitation.

The organic addition agent which is employed for the purpose of the invention is a gluconic acid or a gluconate that is soluble in the acid zinc bath. The addition agent may be added in the form of an aqueous solution of gluconic acid or as a salt thereof, the cation of which does not adversely affect thebath. The gluconic acid addition agent can be added in the form of zinc gluconate previously prepared by the reaction of zinc oxide with gluconic acid. The gluconic acid can be obtained by the oxidation of glucose, or in any other suitable manner, and if the acid is used as such it is preferably employed in the form of an aqueous solution having a concentration of about 50% of gluconic acid.

A second addition agent, which is essential for the purpose of the invention, is an alkylated naphthalene sulfonic acid which may be added in the form of the free sulfonic acid or a soluble salt thereof. The preferred addition agent of this class is isopropyl naphthalene sulfonic acid sodium salt which is available in a commercial form of purity. In the practice of the inconic acid alone nor the alkylated naphthalene sulfonic acid alone will produce the desired image bright zinc plat out of the acid bath even though the lead content of the bath is maintained at less than 5 milligrams per liter. When these two addition agents are employed together, however, a specific and surprising result is obtained. The low lead content is particularly important where the bath is operated at temperatures above about degrees F., and especially in the range of degrees F. to degrees F. At these higher temperatures the presence of dissolved lead in excess of about 5 milligrams per liter of electrolyte causes ridging of the electrodeposit zinc plate. In commercial practice, therefore, where the bath is preferably operated at these higher temperatures and where additional quantities of electrolyte are being added continuously or intermittently to make up for the losses occasioned by the,

operation of the plating process, the maintenance ofa low dissolved lead content in the bath is very important.

. Apart from the necessity to take precautions with respect to the lead content of the bath when operating at the higher temperatures, the temperature of the bath alone does not appear to affect the brilliance of the deposit provided there is sufiicient agitation. Excellent results have been obtained at .roomternperatures (75'degrees R), and equally good results have been obtained at the preferred temperatures above 100 degrees F. and not more than 160 degrees F. In commercial operations the bath becomes heated normally by the current which is passing through it, especially when high current densities are used. It is not ordinarily desirable to operate at temperatures above 160 degrees F. because ofthe operating hazards to the operators. This is especially true where the object being plated is a continuous strip which might break and have to beremoved from the bath. The same difiiculty arises in case individual articles are being plated, as in piece plating, and one or more of these drops fromits rack into the bath. As will be apparent if the bath were heated to a very high temperature it might be necessary to cool it before the strip could be repaired or the article'could be removed from the bath, as the case might be. In the actual, practice of the invention especially good results have been obtained in operating the process at temperatures within the range of 115 degrees F. to 140 degrees F.

Image bright plates can be obtainedin accordance with the invention overa wide pH range of 1 to 5.5. The optimumpH range is 2.5 to 4.0.

The invention can be practiced by adding the addition agents to any of the ordinary a-cid zinc electrolytes which may or may not contain other addition agents. Such zinc electrolytes and their method of preparation aregenerally Well known. Thus, several standard acid zinc plating baths are described in Transactions of the E1ectr0- chemical Society, vol. 80, 1941, page 390. One of the typical standard baths, for example, consists of zinc sulfate, ammonium chloride, sodium acetate, glucose and water. This bath normally produces a grayplate, but by :adding thetwo classes of addition agents in accordance with this invention and maintaining a low lead content as herein described, image bright plates can be obtained. We have determined that the glucose present in this bath in combination with the alkylated naphthalene sulfonic acid added in accordance with this invention will not produce image bright zinc plates. Glucose, therefore, is in no sense the equivalent of 'gluconic .acid .for the purpose of the present invention.

In the past it has been customary to prepare acidzinc electrolytes containing salts other than zinc salts, buffers, glucose or other reducing sugars and various types of organic colloids which have been alleged to have a beneficial effect in the bath. For the purpose of the present invention, however, it has been found that excellent results can be obtained by making the necessary additions to a simple electrolyte consisting of zinc sulfate and water. The zinc content of the bath for the purpose of this invention may vary rather widely, but in continuous plating it'is .preferably within the range of 50 to 150 grams of zinc per liter. In piece plating where speed of operation is not necessarily an important factor itis possible to use a bath having less than 50 grams per liter of zinc. The preferred range of .zinc content in the electrolyte is 50 to 100 grams of zinc per literof bath. Although a zinc content above 150 grams of zinc per liter may be used, it is not essential for the purpose of the invention and is undesirable where current densities in excess of about 100 amperes per square foot are employed.

The invention may be employed over a wide range of current densities. In continuous plating, in accordance with the invention, it"is preferable to operate at current densities within the range of 100 to 250 amperes per square foot. Hull ment used and" the particular article plated. Because of the fact that high current densities may be used in the practice of the process, correspondingly high rates of zinc deposition may be obtained with resultant high production schedules.

Image bright plates may be obtained in accordance with the invention in a varying range of thickness of deposit. The preferred range of thickness in continuous plating of sheets or strips is within the range of 0.000025 to 0.0005. On the other hand, in the plating of wire it may be desirable to deposit much greater amounts of zinc, and this has been done in the practice of the invention. Thus, image bright plates on W re were obtained in thicknesses from 0.002 inch to 0.004 inch (1.2 ounces to 2.5 ounces per square foot). Regardless of the thickness of the deposit image bright deposits have been obtained in the practice of the invention. It will be understood, of course, that where ductility is a factor there may be limits of thickness beyond which a deposit will tend to chip or crack. Such limits can readily be determined in any particular case by routine experiments.

In carrying out the invention it is preferable to maintain a large degree of relative motion between the cathode and the electrolyte because agitation tends to prevent streaking. At the lower temperatures, however, below 100 degrees F., image-bright deposits are obtained without agitation. In most continuous plating processes sufficient agitation is inherently present. If the amount of agitation is sufiicient so that the desired brilliance is obtained without streaking, the rate of agitation can be increased while still maintaining the brilliance of the zinc deposit.

The invention will be further illustrated by the following examples in which the quantities are stated in parts by weight unless otherwise indicated.

Example I An electrolyte was preparedby mixing the following ingredients:

450-grams zinc sulfate 30 grams sodium-acetate 4 cc. of a 25% solution in water of Aerosol OS (a product of American- Cyanarnid Company containing isopropyl naphthalene sulfonic acid sodium salt and 5% inerts) 32 cc, of 50% gluconic acid in water Water sufiicient to make a liter of solution Less than 3 milligrams per liter of soluble or suspended lead.

The pH of this electrolyte'was maintained within thetrange of 2.5 to 4.0.

This'electrolyte produced image bright deposits when electrolyzed at normal current densities common to'the art, such as 25 to 300 amperes per square foot. Higher current densities have been Example II p The following typical experiments illustrate the effect of the presence of lead, using 5 liters of the electrolyte heretofore mentioned with a rotating cathode speed of 205 R. P. M., a cathode circumference of 12 inches, a temperature of 120 degrees F., to which lead had been added to produce the value shown below, using 25 amperes for 15 minutes, or a current density of 100 amperes per square foot.

Lead, Mgs./l. Mgs. Lead Lead i ggg Renli oved gemvedz.

per mp. er en Start Finish Removed Hour of Zinc The last two samples were of a brilliant appearance. The last one only was entirely free from surface imperfections. The previous one has some widely scattered minute pitting on a very small proportion of the surface. Ridging was prevalent in others.

The solution used for the above experiment produced brilliant deposits under identical conditions prior to the addition of lead as lead acetate.

Example III An electrolyte was prepared by dissolving 1920 grams of zinc sulfate (ZnSO4.7H2O) in 8000 cc. of water. This bath was adjusted to a pH of 5.0, filtered and electrolyzed hours at 1 ampere per square foot to remove the lead. The optimum pH was determined by making Hull cell plates at varying pH values. Steel cylinders 6 inches high and having a circumference of 12 inches were then plated from this bath at the optimum pH of 2.5 and at varying rates of agitation and temperatures. A light uniform gray deposit was obtained. A similar series of tests was made with a proprietary composition composed of 2880 grams of zinc sulfate, 240 grams of ammonium chloride, 120 grams of sodium acetate, 960 grams of glucose and enough water to make 8000 cc. at its optimum pH of 2.76. Another similar series of tests was made with a composition composed of 1920 grams of zinc sulfate, 320 grams of zinc gluconate, and enough water to make 8000 cc. of solution at its optimum pH of 2.47. A fourth similar series of tests was made with an electrolyte composed of 1920 grams of zinc sulfate, 320 grams of dextrose and enough water to make 8000 cc. of solution at its optimum pH of 3.0. In no case was a bright deposit produced and there was a complete lack of image brightness in any of the deposits.

One method which has been employed to evaluatethe results of the invention with respect to the brightness of the deposit is to measure the light reflected from a cylinder of the type previously described on a Gardner glossmeter such as is used in the paint industry. The Gardner glossmeter gives a measure of specular gloss of light reflected from an object at an angle of 60 degrees. This is standardized against a black glass plate. aforementioned cylinders gave a reading on this Gardner glossmeter within the range of 110 to 130 or an average reading of 120 based upon numerous tests. The reading on the Gardner glossmeter for a zinc plate deposited on cylinders as previously described in this example was less than 5 in all cases.

. Example IV To the previously described purified zinc sulfate bath containing 1920 grams of zinc sulfate and 8000 cc. of water there was then added varying amounts of zinc gluconate, these amounts being respectively 40 grams, 120 grams, 200 grams, 480 grams and 800 grams. By a series of experiments it was determined that the optimum pH was around 2.5 to 3.5. Hull cells were made from these baths at a current density of 1 ampere per square foot for 10 minutes at 74 degrees F, The Hull cell plates showed that the addition of the zinc gluconate had a pronounced anti-burn effect but in itself did not affect the brightness of the plate.

To the previously described purified zinc sulfate bath .containing 1920 grams of zinc sulfate dissolved in water to produce eight liters of solution there was then added 5, 15, 25, 40, 65 and cc. per gallon of a 25% by weight solution of isopropyl naphthalene sulfonic acid sodium salt. Hull cell plates were made with each of these baths in a manner similar to that previously described and again it was observed that image brightness was not produced. Some slight effect on the grain structure of the deposit was noted, apparently due to a slight decrease in crystal size.

Two acid zinc plating baths were then prepared by (1) adding 1920 grams of zinc sulfate and 320 grams of zinc gluconate to 8000 cc. of water, and (2) adding 480 grams of zinc gluconate to the same quantities of zinc sulfate and water, and adjusting the pH to 2.47. These baths were electrolyzed 17 hours at 1 ampere per square foot for purification. To these baths there was then added varying amounts of isopropyl naphthalene sulfonic acid sodium salt, namely, 5 cc. per gallon, 15 cc. per gallon, 25 cc. per gallon, 40 cc. per gallon, 65 00. per gallon and 80 cc. per gallon of a 25% by weight solution of isopropyl naphthalene sulfonic acid sodium salt. The Hull cell plates made from these baths all showed image brightness in some areas and hence indicated that this image brightness was due to the combined effect of the gluconate and the isopropyl naphthalene sulfonic acid sodium salt. All of these Hull cell plates showed brightness in the lowest current density range. The optimum effect over the widest range of current densities was obtained with 25 cc. per gallon of the isopropyl naphthalene sodium sulfonate solution in the second bath which contained 60 grams per liter of thezinc gluconate.

Example V In order to evaluate the results obtained at different rates of agitation and temperatures, a

A chromium deposit on one of the series: oftests was-made by plating from liters of an electrolyte containing:

240 grams per liter of 'ZnSOrJlI-IQO 40 grams per liter of zinc .gluconate made by dissolving zinc oxide and a hot 50% solution of a gluconic acid together in the chemically equivalent proportions required to form zinc gluconate and cc. per liter of a aqueous solution of 95% isopropyl naphthalene sulfonic acid sodium salt The cathode used consisted of a cylinder of cold rolled steel 6 inches high and 12 inches in circumference. This cylinder was rolled from a strip to a diameter slightly less than that of the cathode holder. It was then possible to slip the cylinder over the cathode holder, permitting contact to be made through a central metallic member of the holder. The cathode holder consisted of .a plastic impregnated block with a central band serving as a contactor for the cathode. The cathode holder was adapted to be driven at variable speeds from about 10 R. P. M. to 205 R. P. M. The anodes and certain baffles were arranged to control any possible cavitation.

The electrolyte was held ina glass jar and the temperature thereof could be controlled by a hot. plate engaging a coppered disc covering the entire surface of the jar. 'The electrolyte in each'case contained less than 5 milligrams of lead. The pH of the bath was adjusted to 3.0 with sulfuric acid and each cylinder was plated for 5 minutes at a current density of 100 amperes per square foot. Each cylinder after plating was then examined under a Gardner glossmeter in order to determine the specular reflection. Although our preferred method of making glcssmeter -tests is to set the glossmeter so that it registers 95 at a 60 degree angle black glass, as a standard in this case theglossrneter was setto register approximately one-half of the usual standard, or 45. This was done .by flowing resistance into the volt meter circuit. The resultant readings, therefore, were proportionately lower than they would have been if taken against the full scale standard.

The results obtained are shown by the following table:

1 Revolutions per minute.

This table shows the effect of temperature and agitation in increasing or decreasing the specunectivity of the resultant plate. All of the ed cylinders in column 1 were image bright. Lin-" ise, all of plated cylinders in column 2 were image bright. The top cylinder in column 3 was bright-but not image bright. The top cylinder in column 4 had a low specular reflection and the same was true with all of the cylinders in column -5 except the lowermost three cylinders. The second and third cylinder from the top of column 4 showed partial brightness with some streaking. The bottom row of cylinders in all columns was image bright. Thus, it appears that at temperatures below degrees neither agitation nor temperature is a factor in producing image brightness in the resultant plate. Both were factors to a varying extent in the other plated cylinders. The temerature was not a factor, however, so long as the agitation was high. This is shown by the fact that all of the plated cylinders in columns 1 and 2 were image bright.

Example VI A cylinder of the type describedin Example V was plated in a rotating cathode apparatus of: the type described in Example V at 85 degrees F. to 9'5 degrees F. and a current density of amperes per square foot for 1 hour and 45 minutes with an electrolyte having the following composition:

240 grams per liter of ZllSO4'7I-I2O l0 grams per liter zinc gluconate 2.5 grams per liter isopropyl naphthalene sulfonic acid sodium salt The bath was adjusted to a pH of 2.5 to 3.0 with sulfuric acid. A coating of approximately 0.0064 inch thick was obtained and the resultant plate was very brilliant.

In a similar manner other examples could be given to illustrate the practice of the invention. The proportions of the various ingredients of the bath can be varied somewhat within limits that can be readily determined by anyone skilled in the art. The lead content should preferably be below that which causes ridging, especially where the bath is to operate at temperatures in excess of 100 degrees F. In most cases it is "preferable to have the lead content less than 2 milligrams per liter of electrolyte. The optimum concentrations of gluconic acid or gluconate in the bath can be determined by preparing Hull cell plates in the manner previously described. In the same way the optimum concentrations of the'alkylated naphthalene sulfonic acid derivative can be determined. Likewise, in a similar manner the optimum pH values can be determined.

The current used in .the various tests described in the examples was direct current. It will be understood that the invention can be employed with other types of current and abnormal wave forms, including pulsating direct current such as may be derived by superimposing alternating current .on direct current or by the use of various types of rectifier arrangements. Where a superimposed current is employed the optimum results can ordinarily be obtained with less agitation.

Although the invention has been specifically described with respect to isopropyl naphthalene sulfonic acid sodium salt, it will be understood that other water soluble alkylated naphthalene sulfonic acids and salts thereof can be employed in the practice of the invention, including the butylpisobutyl, amyl and isoamyl derivatives and the ammonium, potassium and other water soluble alkali. metal and alkaline earth metal. salts of the various .alkylated naphthalene sulfonic acids. Inorganic salts such as are sometimes present in the commercially available alkylated naphthalene'sulfonic acid salts have no adverse efiect on the results. Thus, the isopropyl naphthalene sulfonic acid sodium'salt is commercially pear, however, that the surface tension in itself is a factor in producing the results because numerous other materials which have an efiect in lowering the surface tension have no effect whatsoever in producing an image bright plate. It is evident, therefore, that the surprising results obtained in accordance with the practice of the invention are due to some specific action produced by the. combination of the gluconic acid and the alkylated naphthalene sulfonic acid derivative and the low lead content.

Although the invention is primarily directed toward the preparation of image bright zinc plated materials it will be understood that the bright- .ness of the coating may be varied by controlling the conditions of plating in the manner previously described while still producing desirable coatings. The thickness of the coating, the nature of the surface of the article plated and the type of article are all factors to be considered. Thus, a very thin coat of zinc deposited on a dull surface will not have the brightness of a thicker coat.

The gluconic acid employed in the practice of the invention exists in several forms and the invention contemplates the use of one or more of these forms, or mixtures thereof, including mixtures of the lactone forms. Commercial gluconic acid is available as a 50% aqueous solution of approximately 99% gluconic acid and 1% glucose. The presence of the glucose is undesirable because it tends to build up in the bath to cause gumminess or stickiness. The zinc gluconate described in the examples is substantially free of glucose. This may beprepared by heating a commercial 50% by weight gluconic acid solution to 120 degrees F. sufiiciently long to melt any crystalline lactone that might be present, then adding one mole of lead-free zinc oxide for every two moles of gluconic acid, heating to 170 degrees F. with agitation and holding for minutes until solution is complete. The resultant product is cooled to 120 degrees F. in the liquid phase and poured into large stoneware vessels and cooled until a waxy solid forms containing some supernatant liquor. The liquor which is mainly glucose and water is removed by filtration and the residue is placed in drying trays. At this point it may be washed with ice water to dissolve traces of residual glucose. The product is then dried by heating at temperatures below 250 degrees F. until dry and hard, after which it is ground in a hammermill or other suitable means. The resultant product is stable and non-sticky. The method of preparation and the product are described and claimed in the application of Allan E. Chester, Serial No. 34,127, filed June 19, 1948. The quantities of this material employed with optimum results has been within the range of 25 to 80 grams per liter of electrolyte, or the chemically equivalent proportions of gluconic acid or other bath soluble gluconate.

The term lead content as used herein refers to dissolved or bath soluble lead such as would pass through a filter. The determination for soluble lead is preferably made by using a 10 cc. sample of the plating solution. This is stirred with activated carbon suflicient to remove organic matter. Two (2) grams of activated carbon is usually enough for this purpose. The purified sample is then filtered, washed with water and added to a 250 cc. separatory funnel where it is mixed with a 10 cc. solution of citric acid (containing 500 grams of citric acid made up to a liter with water), 1 cc. of hydroxyl amine hydrochloride (containing 20 grams made up to cc. with water) and 10 cc. of 29% ammonium hydroxide, the resultant pH being around 9.5 to 10. To this sample is then added 60 cc. of 10% potassium cyanide solution and the resultant mixture is shaken in the funnel to mix the various components. There is then added 10 cc. of di-phenyl thiocarbazone (containing 5 milligrams made up to 100 cc. in carbon tetrachloride). The mixture is then shaken again for minute and allowed to settle. The carbon tetrachloride layer which now contains the red lead color is drawn oif into 250 cc. separatory funnel containing 25 cc. of ammonium cyanide solution made by mixing 10 cc. of the 10% potassium cyanide solution with 5 cc. of the 29% ammonium hydroxide and diluting to 500 cc. with distilled water.

Another 10 cc. of the di-phenyl thiocarbazone solution is added to the first funnel and the carbon tetrachloride layer is again drawn off. This is continued until the material which is drawn off no longer contains the red lead color. All of the extracts which have been drawn off are combined into a third separatory funnel containing 50 cc. of the ammonium cyanide solution. This is shaken and allowed to settle. The carbon tetrachloride layer is then drawn off into a 50 cc. volumetric flask and diluted to the 50 cc. mark with carbon tetrachloride.

The quantity of lead in the sample is determined by the use of a Coleman Universal spectrophotometer by determining the transmittance of light having a wave length of 530 millimicrons against a blank. The blank is prepared by adding 20 cc. of distilled water, 10 cc. of the citric acid, 1 cc. of the hydroxyl amine hydrochloride solution, 10 cc. of the ammonium hydroxide solution, 60 cc. of the potassium cyanide solution and 10 cc. of the di-phenyl thiocarbazone solution together in a separatory funnel, drawing off the carbon tetrachloride layer and diluting to 50 cc. with carbon tetrachloride, the procedure being the same as before except that no plating solution is present.

The reading of the photometer is then set to zero with the blank and the reading is taken on the treated sample solution. The lead content is determined by, comparing the transmittancy reading on the sample solution with transmittancy readings of standard lead solutions having a known lead content.

The invention produces surprising and unusual results in the plating of zinc from acid baths. By the practice of the invention plates have been produced which have been comparable to and in some cases even more brilliant than bright chromium plates. So far as is known, these results have not heretofore been attained in this art. Additionally, the invention provides improved performance of acid plating baths, excellent zinc deposits, operation at high current densities and other advantages which will be apparent to those skilled in the art.

Throughout the specification and claims where the addition agents are referred to broadly as a gluconate and an alkylated naphthalene sulfonate it will beunderstood that these expressions comprehend the addition .of the free gluconic acid and free alklated naphthalene sulfonic acid as well as soluble salts thereof to the acid zinc electroplating bath.

Certain of the broader aspects of this invention are covered in a copending application of Allan E. Chester, Serial No. 34,127,, filed of even date herewith.

Various methods of continuously removing. lead from the acid zinc electroplating bath are disclosed in av copending application Serial No. 34,128, filed of even-date herewith.

The invention is hereby claimed as follows:

1. A method of zinc plating which comprises essentially electrodepositing zinc from an acid bath containing zinc sulfate and having a gluconate and an alkylated napthalene sulfonate dissolved in said bath, the quantity of lead insaid :bath being less than milligrams per liter of solution, and carrying out said electrodeposi'tion at temperatures above 100 F. and below 160 E.

2. A method as claimed in claim 1 in which the electrodeposition is carried out with agitation at a temperature within the range of 115 degrees F. to 140 degrees F.

3. A method as claimed in claim 1 in whichthe electrodeposition is carried out at current densities in excess of 25 amp-eres per square foot.

4. A continuous method of electroplating zinc which comprises essentially continuously electrodepositing zinc on a continuously moving strip, sheet or wire as the cathode from an aqueous acid bath comprising essentially dissolved zinc sulfate, zinc gluconate, and a water soluble compound from the group consisting of alkylated naphthalene sulfonic acid and salts thereof while maintaining a lead content in said bath below 5 milligrams per liter of solution and a zinc content within the range of 50 to 150 grams per liter of solution, said acid bath having a pH within the range of 1.0 to 5.5.

5. A continuous method of electroplating zinc which comprises essentially continuously electrodepositing zinc on a continuously moving strip, sheet or wire as the cathode with agitation from an aqueous acid bath comprising essentially dissolved zinc sulfate, zinc gluconate, and a water soluble compound from the group consisting of alkylated naphthalene sulfonic acids and salts :12 thereof, while maintaininga lead content in said bath below 5 .milligrams per liter of solution, a Zinc content within the range of to 150 grams per liter of solution and a temperature between degrees .F. and 160 degrees F., said acid bath having a pH within .the range of 1.0 to 5.5.

'6. A continuous method of electroplating zinc which comprises essentially continuously passing a metal strip, sheet or wire as a cathode on which the zinc is to be deposited, through an aqueous acid bath comprising essentiallyzinc sulfate, zinc gluconate, and isopropyl naphthalene sodium sulfonate, whilemaintaining the zinc contentof said bath within the range of 50 to 1150 grams .of zinc per liter of solution, a lead content less than 2 milligrams of lead per liter of solution, a pH within the range of 2.5 to 4.0, a temperature within the range'of degrees F. to degrees and a current density within the range of 100 amperes per square foot to 250 amperes per square foot.

'7. A method of electroplating zinc which comprises continuously electrodepositing zinc on a continuously moving metal strip, sheet or wire as a cathode from an aqueous acid bath having a pH within the range of 1.0 to 5.5 consisting essentially of dissolved zinc sulfate, zinc gluconate and isopropyl napthalene sodium sulfonate, said bath having a zinc content within the range of 50 to grams per liter of solution, a lead content below 5 milligrams per liter of solution, a zinc gluconate" content Within -the range of .25 to 80 grams per liter of solution, and an isopropyl naphthalene sodium 'salt content within the range of 1.5 to 2.5 grams per liter of solution, and maintaining the temperature between 100 degrees and degrees F.

ALLAN E. CHESTER. RAY F. MAIN. HAROLD F. HANSEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 694,658 Meurant Mar. 4, 1992 905,837 Broadwell Dec. 8, 1908 1,818,229 Lutz et al Aug. 11, 1931 2,451,426 Bair et al Oct. 12, 1948 2,485,563 Chester et a1 Oct. 25, 1949 

1. A METHOD OF ZINC PLATING WHICH COMPRISES ESSENTIALLY ELECTRODEPOSITING ZINC FROM AN ACID BATH CONTAINING ZINC SULFATE AND HAVING A GLUCONATE AND AN ALKYLATED NAPHTHALENE SULFONATE DISSOLVED IN SAID BATH, THE QUANTITY OF LEAD IN SAID BATH BEING LESS THAN 5 MILLIGRAMS PER LITER OF SOLUTION, AND CARRYING OUT SAID ELECTRODEPOSITION AT TEMPERATURE ABOVE 100* F. AND BELOW 160* F. 